Distributed energy generation, such as solar and wind, are gaining popularity, however, the ability to store that energy at the place of generation (i.e., onsite) is lagging. Storage is necessary in off-grid applications where energy generated onsite is often the only energy available. However, even in grid-tied applications, storage can be important to the overall value proposition of solar for several reasons. First, in jurisdictions that don't enable customers to back feed power to the grid when their generated power exceeds their onsite demand (net metering), storage provides a way to save and use that power later when onsite generation is no longer possible (e.g., after sunset). Second, even in jurisdictions that allow net metering, storage may be useful as a back-up power source during grid outages, and also as a way to help customers and/or utilities shave the peaks off of their power demand curves by discharging storage devices during periods of peak demand.
In the past, the only practical way to store locally generated energy was using deep-cycle lead acid batteries. These batteries are large, heavy and operate at relatively low voltages (e.g., 12-48 volts). Because they are low voltage and very high current battery modules, a transformer must be used to provided galvanic isolation, and a DC/DC buck-boost converter must be used to boost the voltage to a suitable level for inversion to alternating current or to step-down the voltage when charging the battery by rectifying the alternating current. A certain non-trivial percentage of efficiency gets lost during charging and discharging as power losses in the transformer and the DC/DC converter stages.
Recently, higher voltage lithium-ion battery packs that have been developed to provide improved performance over lead-acid batteries within a smaller form factor. These batteries can range in voltage from 48 volts all the way up to 200-800 volts and possibly even higher depending on the cell configuration of the battery modules (i.e., the number of series and parallel groupings of cells). Although they are able to store more energy per unit of mass (energy density) than lead-acid batteries, and also operate at much higher voltages, they still suffer from charging and discharging power losses attributable to the transformer and DC/DC converter stages.
Therefore, there exists a need for techniques to reduce or eliminate efficiency losses associated with battery packs.